Hal,

For a 5lb 3D plane, use a power system that can deliver around 150 watts per pound (or 150w/lb).

This means that you need a motor that can handle at least 5 x 150w or 750 watts.

Using and outrunner like an [link=http://www.hobby-lobby.com/brushless-axi4130.htm]AXI 4130/16[/link] motor can provide this power for a good cost value. The outrunner motor is also maintenance free as there is no gearbox to lubricate or pinion gear to worry about slipping.

A 20 cell NiCd or NiMH setup with a 14x7 or 15x7 APC e-prop is a good starting point. The combination AXI 4130/16 motor and Jeti 77-amp Opto ESC ([link=http://www.hobby-lobby.com/brushless-combo-axi.htm#413016]PM413016C[/link]) can be purchased at Hobby Lobby for only $329. This is less than half your $700 budget.

I would start out with two 10-cell battery packs, and, if you like the result, switch to two 3-cell Lithium packs.

Good luck!